The application of coating materials using electrostatic spraying techniques has been practiced in industry for many years. In these applications, the coating material is discharged in atomized form and an electrostatic charge is imparted to the atomized particles which are then directed toward a substrate maintained at a different potential to establish an electrostatic attraction for the charged atomized particles. In the past, coating materials of the solvent-based variety, such as varnishes, lacquers, enamels and the like, were the primary materials employed in electrostatic coating applications. The problem with such coating materials is that they create an atmosphere which is both explosive and toxic. The explosive nature of the environment presents a safety hazard should a spark inadvertently be generated, such as by accidentally grounding the nozzle of the spray gun, which can ignite the solvent in the atmosphere causing an explosion. The toxic nature of the workplace atmosphere created by solvent coating materials can be a health hazard should an employee inhale solvent vapors.
As a result of the problems with solvent-based coatings, the recent trend has been to switch to water-based coatings which reduce the problems of explosiveness and toxicity. Unfortunately, this switch from electrostatically spraying solvent-based coatings to those of the water-based type has sharply increased the risk of electrical shock, which risk was relatively minor with solvent-based coatings. The risk of electrical shock is occasioned in the use of water-based coatings due to their extreme electrical conductivity, with resistivities of such water-based coatings often falling within the range of 100 to 10,000 ohm centimeters. This is in contrast to resistivities of 200,000 to 100,000,00 ohm centimeters for moderately electrically conductive coatings such as metallic paint, and resistivities exceeding 100,000,000 ohm centimeters for solvent-based lacquers, varnishes, enamels and the like.
The relative resistivity of the coating material is critical to the potential electrical shock which may arise during an electrostatic coating operation. With coating materials which ar either not electrically conductive or only moderately electrically conductive, the column of coating material which extends from the charging electrode at the tip of the coating dispenser through the hoses leading back to the supply tank has sufficient electrical resistance to prevent any significant electrostatic charging of the material in the supply tank or the tank itself. However, when coating material is highly electrically conductive, as are water-based coatings, the resistance of the coating column in the supply hose is very low. As a result, a high voltage charging electrode located in the vicinity of the nozzle of the coating dispenser electrostatically charges not only the coating particles, but the coating material in the hose, the coating material in the supply tank and the supply tank itself. Under these circumstances, operating personnel inadvertently coming into contact with an exposed supply tank, or a charged hose, or any other charged part of the system, risk serious electrical shock unless such equipment is grounded to draw off the electricity. If the equipment is indeed grounded at any point, however, the electrostatics will not function because the high voltage charge would be conducted away from the coating dispenser electrode as well.
One of the methods for reducing the electrical shock problem is disclosed, for example, in U.S. Pat. No. 3,971,337 to Hastings, which is owned by the same assignee as this invention. The Hastings patent discloses an apparatus for electrostatically isolating the supply tank which is connected to the coating dispenser. While this device is satisfactory for batch operations, it does not readily lend itself to continuous painting lines, i.e., applications wherein an essentially continuous supply of coating material must be provided over a period of time.
This problem has been addressed in apparatus of the type disclosed, for example, in U.S. Pat. No. 4,313,475 to Wiggins. In apparatus of this type, a "voltage block" system is employed wherein an electrically conductive coating material is first transmitted from a primary coating supply into a transfer vessel which is electrically isolated from one or more electrostatic coating dispensers. When filled with coating material, the transfer vessel is first disconnected from the primary coating supply and then connected to an inventory tank, which, in turn, is connected to the coating dispensers. The coating material is transmitted from the transfer vessel into the inventory tank with the transfer vessel disconnected from the primary coating supply, to fill the inventory tank with coating material for subsequent transfer to the coating dispensers. After the inventory tank is filled, the transfer vessel is disconnected from the inventory tank and connected back to the primary coating supply to receive another quantity of coating material so that the coating operation can proceed essentially continuously.
The coating material supplied from the inventory tank in the Pat. No. 4,313,475 system is subjected to a high voltage electrostatic charge, upstream from the coating dispensers, so that charged coating material is supplied to each of a number of coating dispensers for deposition onto a substrate. In the event different colors are to be dispensed from such system, a color changer is provided which fills the inventory tank with a desired color for subsequent transmittal to the transfer vessel supplying the coating dispensers.
Current National Fire Protection Code provisions for electrostatic spray painting require the electrostatics to each manually operated coating dispenser to be shut down when the trigger of the gun is released. One problem with systems of the type disclosed in the Wiggins Pat. No. 4,313,475 is that no provision is made to electrically isolate each of the coating dispensers when not in use, i.e., when the operator releases the trigger of the dispenser. As mentioned above, a high voltage electrostatic charge is applied to the coating material discharged from the transfer vessel of the Pat. No. 4,313,475 system upstream from the coating dispensers so that the coating material and, hence, the coating dispensers, all remain charged regardless of whether or not the dispensers are in use. While this system may be satisfactory for automatically operated coating dispensers, the National Fire Protection Code requirements for manually operated spray guns are not met by the Pat. No. 4,313,475 system.
Another problem with systems of the type disclosed in the Wiggins Pat. No. 4,313,475 is that the color changer associated with such system is located upstream from the inventory tank. In order to change colors, essentially the entire system must be cleaned, i.e., the inventory tank, transfer vessel, coating dispensers, and all the lines interconnecting these elements. This is a time-consuming and cumbersome operation which is unacceptable in applications wherein rapid color changes are required.
A still further problem with systems of the type disclosed in the Wiggins Pat. No. 4,313,475 is that they cannot be used with coating materials whose application characteristics are improved when dispensed at elevated temperatures. In systems of this type, it is not feasible to use a coating material heater because no provision is made to recirculate the coating material from the coating dispensers back to the source when the coating dispensers are not in use. Absent recirculation, the coating material could not be held at sufficient temperature if the spraying operation were interrupted or discontinued for a period of time. Additionally, in systems of the type disclosed in Wiggins Pat. No. 4,313,475, any heater utilized would have to be positioned in the loop between the source of coating material and inventory vessel to isolate the heater from the electrostatic power supply and avoid grounding of the system. At this location, the heater is physically removed from the coating dispensers and could not effectively maintain temperature of the coating material unless the system was always operated continuously.